The present invention relates to the production of coated abrasives and particularly to the production of coated abrasives carried on a film backing. Such products are typically used for fine finishing applications. In typical examples of such applications the abrasive is in the form of a sheet wound on a roll that is unwound from the roll and supplied to the grinding station where it is held against the workpiece to be ground using some sort of precession shaped tooling or shoes. After contact with the workpiece, the sheet is wound up on a take-up roll to ensure constant tension. A polymer film does not usually have very good friction qualities and if an untreated back surface were in contact with the shoes excessive slipping would occur and there would be wrapping of the film around the bearing suface and ultimately, breakage of the film. This results in extensive down-time for the manufacturer and is regarded as extremely undesirable.
The preferred film is often a polyester which has a unique blend of uniformity, non-compressibility, resistance to water and high tensile and tear strength. However it also has in high degree the problems of slippage referred to above which can lead to failure of proper indexing and even film breakage.
For this reason a film backing is usually supplied with a friction promoting surface. This surface is typically provided by abrasive particles in a binder. For reasons of speed of production, it is often preferred to use a radiation curable binder but these come with a practical problem. Radiation-curable binders are typically 100% reactive, that is there is no carrier medium which must be evaporated before the cure of the binder resin. Thus there is little shrinkage involved upon cure and the amount of the filler particles projecting above the binder layer is strictly dependant on the volume percent represented by the filler particles in the composition. The amount of abrasive that can be incorporated is limited by the rheology of the mixture as well as its viscosity which both impact the coatability of the binder/filler mixture. If too little is used however this can lead to the particles being buried in the binder with only relatively small amounts showing above the binder surface. This results in unsatisfactory frictional characteristics and can lead to slippage, film breakage, excessive tooling or shoe wear and tooling contamination.
In a preferred product the friction coating allows a pattern of rapid, slip/stick events to occur such that, overall, the pressure remains relatively constant. However all too often with conventional back coatings the frictional characteristics degrade with time. This occurs as the relatively few exposed grits are worn down and slipping increases. Slipping means relative movement of the backing with respect to the surfaces on which the back surface of the film bears and consequent wearing away of these surfaces.
A backing has now been devised that avoids the above problems and allows the pressure of the belt against the workpiece to be held reasonably constant with minimal slippage and therefore wear on the members against which the back surface bears during the finishing operation.